Investigations continue into the relationship of genetic and non-genetic factors in mouse mammary tumorigenesis. Interest is in the different lines of naturally occurring mammary tumor virus (MTV), and the genetically variable inbred mouse strains. Work is also concerned with the manner of transmission of the various MTV's, i.e. whether by female or equally well by either parent. Finally, strains of mice that develop hormone responsive mammary gland tumors and hyperplastic alveolar nodules (HAN), from which malignant mammary tumors arise are also being studied.